(1) Field of the Invention
The present invention relates to an apparatus for controlling the position of a light beam to follow an information track on an information storage disk of a recorded information reproducing system, and, more particularly, to a light beam position control apparatus using a push-pull tracking servo method.
(2) Description of the Prior Art
In general, a recorded information reproducing system is equipped with a tracking servo device to center a light beam on an information track on an information storage disk, such as an optical disk.
There are various methods for such control, e.g., the one-beam method, wobbling method, and three-beam method. One of the one-beam methods is the push-pull method. This method utilizes the light beam reflected and diffracted by pits on an optical disk; the pit depth being one-eighth the wavelength of the light beam from the light source in order to obtain the best diffraction. The far field of the reflected light beam is sectioned into two half-circles each of which is provided with a photoelectric transducer. These transducers, which together constitute a photodetector, are composed of photodiodes which generate signals indicative of the intensity of light in the half-circle. Deviation of the light beam from the information track creates a difference in the intensity of light in the two half-circles. Thus, a difference between the signals from the photodiodes indicates deviation of the light beam from the information track, in other words, indicates a radial error of the light beam. This difference of the signals from the photodiodes is used as a tracking servo signal in the push-pull tracking servo method. It is important to keep the reflected light beam centered on the photodetector to detect only the difference in the intensity of light in the two half-circles.
In the conventional push-pull tracking servo method, however, the position of the reflected light beam on the photodetector wobbles. The reason for this is that the optical axis of the light beam projected onto the optical disk through an objective lens is not always kept vertical with respect to the optical disk surface due to the displacement of a light-deflection mechanism, i.e., a galvano-mirror or a movable objective lens. Displacement of the galvano-mirror or objective lens moves the light beam projected onto the optical disk surface transversely with respect to the information track. If the light beam is not vertically projected onto the optical disk surface, the reflected light beam from the disk does not follow the same path of the beam projected onto the disk, causing the reflected light beam not to be centered on the photodetector. The false signal due to the deviation of the reflected light beam onto the photodetector combines with the correct tracking servo signal and makes accurate tracking servo operation difficult.
As a result, according to the conventional method, the amount of displacement of the light-deflection mechanism has to be limited so that the optical axis of the light beam projected onto the disk surface does not incline too much from a vertical position with respect to the disk surface. This makes the controllable radical error of the tracking servo mechanism extremely small.